1. Field of the Invention
The present disclosure relates to a density control technique in an image forming apparatus.
2. Description of the Related Art
An image forming apparatus using an electrophotographic scheme is required to have output image density stability and tone stability. U.S. Pat. No. 5,752,126 and U.S. Pat. No. 5,583,644 each disclose an arrangement in which a test pattern is formed on an image carrier and the density of the formed test pattern is read using a sensor or the like to adjust an image forming condition or generate a tone correction table, thus stabilizing the image quality.
The density of an image formed by the image forming apparatus depends not only on the image forming condition such as developing contrast potential but also on the charge amount of the developer itself. For example, when the consumption of the developer increases due to the use status of the user, the developer in the developing unit is used for image formation without being sufficiently frictionally-charged. This increases the density of the formed image. When the consumption of the developer decreases due to the use status of the user, on the other hand, the charge amount of the developer in the developing unit increases more than expected, because of frictional electrification. This decreases the density of the formed image. For example, if the charge amount of the developer in the developing unit keeps increasing, density control is performed to keep increasing the output image data value corresponding to the input image data value in the tone correction table, in order to compensate for the decrease in density. However, since the output image data value has a maximum value, if the charge amount of the developer in the developing unit keeps increasing, the tone correction table generated in the density control may convert all input image data values not less than a predetermined value to the same maximum output image data value. The tone characteristics cannot be maintained in such a case. If the charge amount of the developer in the developing unit keeps decreasing, density control is performed to keep decreasing the output image data value corresponding to the input image data value in the tone correction table. In such a case, jaggies may appear in line portions of the formed image.